pregnancy

Why Textbooks May Need to Update What They Say About Birth Canals

A new study shows that the structure of the human pelvis varies between populations, which could have implications for how babies are birthed.

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A comparison of two human pelvises: The top is more oval in shape, representative of Europeans, North Africans and Native Americans. The bottom is more circular, representative of sub-Saharan Africans and Asians.CreditCreditLia Betti

Look up the term “pelvic canal” in the typical anatomy or obstetric textbook, and you likely will find a description such as this: “Well-built healthy women, who had a good diet during their childhood growth period, usually have a broad pelvis.”

Such a pelvis, the text continues, enables “the least difficulty during childbirth.”

But such characterizations have long been based on anatomical studies of people of European descent. In reality, the structure of the pelvic canal, the bony structure through which most of us enter the world, varies tremendously between populations, according to a new study in Proceedings of the Royal Society B.

Limited prescriptions of what constitutes a “normal” pelvis or birthing process might lead doctors to perform unnecessary interventions — like induced labor, cesarean sections or the use of forceps — which can further exacerbate harm, said Lia Betti, an anthropologist at the University of Roehampton in London, and the study’s lead author.

“What worries me is that doctors come out of school thinking of the European model of the pelvis,” Dr. Betti said. In the early 1900s, this led to “horrific situations” in which American doctors used forceps on black mothers, trying to force babies to align with “the rotation pattern for a European classical pelvis,” she added.

Modern humans have narrow pelvises compared to the size of babies’ heads. That discrepancy contributes to higher rates of birthing complications in humans than in other primates.

Factors such as how long it takes a baby to progress through the canal, or which direction the head is facing on delivery, could change depending on pelvic shape.

There is no accepted explanation for why the human pelvis leaves such little room for childbirth. Dr. Betti and her colleague Andrea Manica, of the University of Cambridge, set out to study a classic if highly-contested explanation known as the “obstetrical dilemma” hypothesis.

The dilemma posits that as our species evolved and began walking upright, the width of the human pelvis narrowed, enabling the body’s weight to stay closer to its center of gravity. But as humans also developed bigger brains, it became harder for a fetus’s skull to squeeze through that tight channel.

Dr. Betti is skeptical of this explanation, and thinks that other possibilities, such as modern diets or the need to support internal organs, could help explain the mismatch between pelvis and fetus.

To explore the idea, she and Dr. Manica measured 348 skeletons from around the world. They found that pelvic shape varied enormously, even more than measures of leg, arm and general body proportion that are known to vary significantly between populations. That was “remarkable and unexpected,” the researchers wrote.

Mostly, they found, pelvic shape varied along lines of geographic ancestry. People of sub-Saharan origin generally had the deepest pelvises back-to-front, while Native Americans had the widest side-to-side. Europeans, North Africans and Asians fell in the middle of the range.

Birth-canal shape also varied markedly within populations, although the variation decreased the farther a population originated from Africa. That finding is consistent with others indicating that a population’s genetic diversity declines the farther it moves from the cradle of humankind

Most of that variation in pelvic shape stemmed from random fluctuations in gene frequency, although natural selection seems to have played a minor role as well, Dr. Betti said. The top of the birth canal is slightly wider in populations from colder climates, perhaps to help make the body stockier.

The variation observed by Dr. Betti suggests that pelvic shape is not so strictly controlled. And if pelvic shape is highly variable across populations, it’s likely “that the birthing process is also highly variable,” said Helen Kurki, an anthropology professor at the University of Victoria in Canada.

These findings challenge the idea “that there is one ‘right’ way to birth a baby,” Dr. Kurki said, and suggest that a more individualized approach to childbirth might be better.

Although people differ from one another anatomically, Dr. Betti said, her research suggests that those differences are not always functional.

“If you look at the shape of the birth canal in different people, it could be tempting to think it’s adapted to give birth to babies with differently-shaped heads, or something like that,” she said.

“In fact, the differences are mostly by chance, which I think is beautiful. Sometimes human variation is just random.”

Childbirth can clearly be a scary prospect. For women who have not given birth before, it is the great unknown. Research into women’s concerns and fears suggests that women may be anxious about the risk of injuries or complications, pain, their ability to give birth, losing control, and interactions with health professionals.

Even women who have given birth before may have similar worries – as every birth is different. But they may also have specific concerns if they had a difficult previous birth experience. This suggests that not only is it normal for women to be worried or anxious about birth, but that it would be unusual if they were not.

But of course, anxiety about birth occurs along a continuum. This ranges from women who are a little bit worried, to those who have developed a true phobia of birth. For some women, this phobia – known as tokophobia – is so severe that they never become pregnant or, if they do, they may decide to terminate the pregnancy.

Severe fear

There are interventions which can be effective for severe tokophobia. But women will only benefit from them if they feel able to disclose their severe fear (and are taken seriously) – or if healthcare professionals are able to identify them. This does not just require training and increased awareness of tokophobia, but also appropriate screening tools and care pathways that ensure women receive timely and appropriate treatment.

At the University of Hull we have been working with local services for the last decade to ensure that women with perinatal mental health problems receive the care and support they need. Together with mental health practitioners, midwives and health visitors we are now developing a pathway for women with tokophobia.

Impact of social media

There is some research evidence that other people’s negative birth stories may increase the fear of birth for some women. But on the other hand, many women find it helpful to talk about their experiences. And research shows that peer support and sharing stories about challenging experiences can help to reduce feelings of isolation and provide validation.

Clearly, there is a tension here between the needs of these two groups of women – those who find it therapeutic to talk about their birth experiences and those whose fears may be increased by reading these stories. Contrary to some media reports, this is not about telling women to “shut up about childbirth”, but it is important to be mindful of the impact sharing may have on others.

Then there is also the issue that women who share traumatic experiences of birth on social mediamay have an unmet need for professional support – which would most likely be a more effective way of helping them cope with these experiences.

The right care

So while it’s clear that some anxiety and worry around birth is normal and to be expected, it’s also important that women with a more severe fear of birth feel able to talk about their concerns – rather than being told that it’s normal to be worried and that everything will be OK. This is important, because women who are experiencing a severe, overwhelming fear of birth will not be reassured by being told that everybody gets a little anxious. What they need is supportive, appropriate and timely care.

That’s not to suggest every pregnant woman who ever expresses any concerns about birth needs to be treated for fear of birth, though pregnant women with raised anxiety levels may benefit from interventions even if they do not suffer from tokophobia. Anxiety in pregnancy has been linked to a number of negative effects on mothers and babies. And early intervention can be crucial in preventing this leading to more serious problems.

But what’s most important, is that all women receive appropriate care and support – including women who have a severe fear of birth and those who have experienced traumatic births. Providing high quality care for all women should diminish the chances of women developing tokophobia after their first birth and help to reduce the amount of negative stories being shared.